In recent years, the electrophotographic technique has been utilized not only to the field of copying machines but also to the fields of printing materials, slide films or microfilms for which the photographic technique was used so far, and it is applied also to high speed printers using lasers, light emitting diodes (referred to simply as LED), or cathode ray tubes (referred to simply as CRT). In the electrophotographic process in which images are formed using the electrophotographic technique, image formation is carried out as described below. At first, the surface of an electrophotographic photoreceptor (hereinafter also referred to simply as ‘photoreceptor’) is charged to a predetermined potential and exposure is applied in accordance with image information to the charged surface of the photoreceptor thereby forming electrostatic latent images. The thus formed electrostatic latent images are developed with a developer containing a toner and the like and visualized as toner images. Images are formed by transferring the toner images from the surface of the photoreceptor to a recording medium such as paper and fixing the transferred images. Along with development for the application range of the electrophotographic technique, demands for the electrophotographic photoreceptor have become severe and versatile more and more.
An electrophotographic photoreceptor comprises a conductive support formed of a conductive material and a photosensitive layer on the conductive support. As the electrophotographic photosensitive material, inorganic photoconductors having photosensitive layers comprising, as the main ingredient, inorganic photoconductive materials such as selenium, zinc oxide or cadmium have been used generally. While the inorganic photoreceptors have a basic characteristic as the photoreceptor to some extent, they involve a problem that the film formation of the photosensitive layer is difficult, the plasticity is poor and the manufacturing cost is expensive. Further, the inorganic photoconductive materials are generally highly toxic and impose large restriction in view of manufacture and handling.
More specifically, typical electrophotographic photoreceptors using inorganic type photoconductive material (hereinafter referred to as “inorganic photoreceptor”) include, for example, a selenium photoreceptor using amorphous selenium (a-Se) or amorphous selenium arsenide (a-AsSe), a zinc oxide photoreceptor in which zinc oxide (ZnO) is dispersed together with a dye as a sensitizer in a binder resin, a cadmium sulfide photoreceptor in which cadmium sulfide (CdS) is dispersed in a binder resin, and an amorphous silicon photoreceptor using amorphous silicon (a-Si) (hereinafter referred to as “a-Si photoreceptor”). However, the inorganic photoreceptor involves the following drawbacks. The selenium photoreceptor and the cadmium sulfide photoreceptor involve a problem in view of heat resistance and store stability. Further, since selenium and cadmium are toxic to human bodies or environments, the photoreceptors using them have to be recovered and discarded properly after use. Further, the zinc oxide photoreceptor has a drawback of low sensitivity and low durability and it is scarcely used at present. Further, while the a-Si photoreceptor that attracts attention as an in organic photoreceptor causing no public pollution has advantages such as high sensitivity and high durability, it involves a drawback that it is difficult to form a photosensitive layer into a uniform film and tends to cause image defects since it is manufactured by using a plasma chemical vapor deposition (simply referred to as CVD). Further, it has also a drawback that the productivity is low and the production cost is high.
Further, development has been proceeded in recent years for the photoconductive material used for the electrophotographic photoreceptor and organic photoconductive materials, that is, organic photoconductors (simply referred to as OPC) have been often used instead of the inorganic photoconductive materials used so far.
The organic photoconductive materials have been studied and developed generally and they are not only utilized for the electrophotographic photoreceptor but also have been started to be applied, for example, to electrostatic recording devices, sensor materials or organic electro luminescent (simply referred to as EL) devices.
Since the organic photoreceptor having a photosensitive layer using the organic photoconductive material (hereinafter also referred to sometimes as “organic photoreceptor”) has advantages that the film forming property of the photosensitive layer is favorable, it is also excellent in flexibility, reduced in weight, and excellent in transparency, and can be easily used for the design of a photoreceptor showing favorable sensitivity to a wide range of wavelength regions by an appropriate sensitizing method, it has been developed gradually as a main stream of the electrophotographic photoreceptor. While the organic photoreceptor involves some problems in view of sensitivity, durability, and stability to the environment, it has many advantages compared with the inorganic photoreceptor with respect to toxicity, production cost, degree of freedom for the design of material, etc. Further, it has also an advantage that the photosensitive layer can be formed by an easy and inexpensive method as typically represented by a dip coating method. In view of the advantages described above, the organic photoreceptor has gradually become predominant in the electrophotographic photoreceptor. Studies have been conducted particularly in recent years, and improvement for sensitivity and durability has been intended and the organic photoreceptor has been used at present as an electrophotographic photoreceptor excepting for special cases.
Particularly, the performance of the organic photoreceptor has been improved remarkably by the development of a function separated type photoreceptor in which the charge generating function and the charge transportation function are shared respectively on separate materials. Further, since the function separated type photoreceptor has a photosensitive layer in which a charge generation layer containing a charge generation substance for charge generating function and a charge transportation layer containing a charge transportation substance for charge transportation function are stacked, it has also an advantage that the range for selecting the materials for the charge generation material and the charge transportation substance is wide, and that an electrophotographic photoreceptor having arbitrary optional characteristics can be manufactured relatively easily. The charge generation layer and the charge transportation layer are usually formed with the charge generation substance and the charge transportation substance being dispersed in a binder resin as a binder.
For the organic photoconductive material used for the charge generation substance of the function separated type photoreceptor, various substances such as phthalocyanine pigment, squarylium dye, azo pigment, perylene pigment, polynuclear quinone pigment, cyanine dye, squaric acid dye, and pyrylium salt dye have been studied and various substances of excellent light fastness and having high charge generating ability have been proposed.
On the other hand, as the organic photoconductive material used for the charge transportation substance, various compounds such as pyrazoline compounds (for example, refer to Japanese Examined Patent Publication JP-B2 52-4188 (1977)), hydrazone compounds (for example, refer to Japanese Unexamined Patent Publication JP-A 54-150128 (1979), Japanese Examined Patent Publication JP-B2 55-42380 (1980), Japanese Unexamined Patent Publication JP-A 55-52063 (1980)), triphenylamine compounds (for example, refer to Japanese Examined Patent Publication JP-B2 58-32372 (1983) and Japanese Unexamined Patent Publication JP-A 2-190862 (1990)), and stilbene compounds (for example, refer to Japanese Unexamined Patent Publications JP-A 54-151955 (1979) and JP-A 58-198043 (1983)). Recently, pyrene derivatives, naphthalene derivatives and terphenyl derivatives having a condensed polynuclear hydrocarbon system at the center nuclei (for example, refer to Japanese Unexamined Patent Publication JP-A 7-48324 (1995)), etc. have also been developed.
The charge generation substance and the charge transportation substance are usually used in a manner of being dispersed or dissolved in a binder resin as a binder in order to ensure the mechanical strength of the photoreceptor. As the resin used for the binder resin, various resins such as polymethyl methacrylate resin, polycarbonate resin, and polyester resin have been proposed.
Performances required for the electrophotographic photoreceptor in the electrophotographic process are, for example, high surface potential when it is charged, high carrier retention ratio, high light sensitivity, and less fluctuation of such electric characteristics under all circumstances. Further, it is also demanded that the photosensitive layer has high film strength, is excellent in wear resistance when used repetitively, and high stability of the characteristics throughout the period of use, that is, high durability. Further, while the photosensitive layer is generally formed by coating a coating solution obtained by dissolving or dispersing the charge generation substance, the charge transportation substance and the binder resin in an appropriate solvent on an electroconductive substrate, it is demanded that the coating solution is stable both physically and chemically in order to improve the production efficiency of the photoreceptor.
Among the requirements described above, the durability is a principal subject of the organic photoreceptor put to practical use. The organic photoreceptor put to practical use involves a problem of tending to cause scraping of film in the photosensitive layer, and change of characteristics such as lowering of the charge potential and increase of the residual potential attributable to electrical change or chemical change. They are caused mainly by the insufficient printing resistance of the photosensitive layer and denaturation and decomposition of an organic photoconductive material such as the charge transportation substance contained in the photosensitive layer by the exposure of the photoreceptor to light or ozone and nitrogen oxide in the photographic process of repeating the steps of forming electrostatic latent images by charging and exposure, transfer of toner images to a recording medium and elimination of the toner remaining on the surface of the photoreceptor by a blade or the like. Accordingly, the role of the binder resin and the charge transportation substance contained mainly in the photosensitive layer as the surface layer of the photoreceptor is extremely important.
As the binder resin, among the resins described above, 2,2-bis(4-hydoroxyphenol)propane (common name; bisphenol A) or bisphenol A polycarbonate resin using the derivatives as the raw material is used mainly. However, the electrophotographic photoreceptor using the bisphenol A polycarbonate resin as the binder resin involves the following drawbacks. Since the bisphenol A polycarbonate resin has high crystallinity, the solution tends to cause gelation and the coating solution becomes no more usable in a short period of time in a case of forming a film by coating. Further, in a case of using the film by coating, when the prepared photoreceptor is used in an electrophotographic apparatus such as a copying machine since the crystallized polycarbonate resin sometimes precipitates to the surface of the formed film, toner is deposited to convex potions formed by crystallization of the polycarbonate resin, the toner at the portions is not completely removed by cleaning but sometimes remains to cause image defects due to cleaning failure. Further, the surface of the photoreceptor tends to be injured and the photosensitive layer tends to be worn by being rubbed in the developing step or cleaning step in the electrophotographic apparatus. That is, the durability is low.
In order to solve the drawbacks, various resins have been proposed. For example, a copolymer of bisphenol A and other molecules has been studied. However no sufficient result has yet been obtained. Further, a polycarbonate resin having a novel specified structure has been proposed (refer to Japanese Examined Patent Publication JP No. 3258537).
Further, in order to compensate the drawback of various resins, use of two or more kinds of resins in admixture has been studied. For example, it has been proposed mixing of a bisphenol A polycarbonate resin and a bisphenol Z polycarbonate resin (refer to Japanese Examined Patent Publication JP-B2 3-49426 (1991) or mixing of a polycarbonate resin synthesized from an asymmetric diol and a polycarbonate resin synthesized from an asymmetric diol (refer to Japanese Unexamined Patent Publication JP-A 6-317917 (1994)). However, for the improvement of the durability of the photoreceptor, mere improvement for the binder resin and the charge transportation substance independently of each other is still insufficient and improvement has to be made also taking the interaction and the compatibility between both of them into consideration.
Further, use of a polyarylate resin has been studied. While the polyarylate resin has a structure similar with the polycarbonate resin, there is a difference among the characteristics of the photoreceptors using the resins. While it has been known that the photoreceptor using the polyarylate resin is excellent in the mechanical stress, when the polyarylate resin is used as the binder rein for the charge transportation layer, it results in a drawback of tending to cause lowering of the potential retaining ratio or increase of the residual potential depending on the structure of the charge transportation substance to be used.
On the other hand, as transfer means of the electrophotographic apparatus forming images by electrophotography, a transfer charger that applies electric charges to a recording medium to generate an electric field for attracting the toner on the surface of the photoreceptor thereby transferring toner images on the surface of the photoreceptor to the recording medium has been used. However, in a case of conducting transfer by the transfer charger, since the recording medium is merely deposited electrostatically but not fixed to the photoreceptor at the transfer portion, this tends to cause a phenomenon referred to as transfer deviation in which toner images can not be transferred accurately to the recording medium during transfer. While the phenomenon was less elicited in electrophotographic apparatus of an analog system type or at low resolution, the problem of the transfer deviation has become conspicuous accompanied to digitalization and increasing resolution in resent years.
In order to prevent the transfer deviation, a transfer roller has often been used instead of the transfer charger. In a case of conducting transfer by using the transfer roller, the transfer roller as a charging member of a roller shape constituted with electroconductive rubber or the like is urged against the photoreceptor from the recording medium on the side opposite to the contact surface of the photoreceptor, thereby applying electric charges in a state where the photoreceptor and the recording medium are in press contact with each other. Use of the transfer roller can prevent the transfer deviation. However, in a case where press contact is weak, a portion of the toner images remains without being transferred to the recording medium tending to cause blanking where white portions are formed in the images, so that it is necessary to increase the pressing force. Increase of the pressing force results in an additional problem that the scraping amount of the photosensitive layer increases due to friction between the recording medium and the transfer roller. Accordingly, higher mechanical strength is required for the photoreceptor more and more.
With the requirement, various improvements have been attempted for the photoreceptor using the polyarylate resin of excellent mechanical strength described above. For example, it has been proposed a photoreceptor using a polyarylate resin and other resin in admixture (refer to Japanese Unexamined Patent Publications JP-A 10-20517 (1998) and JP-A 2000-221722), a photoreceptor in which the surface smoothness and the stabilization of electric characteristics are made compatible by the mixing of the polyarylate resin and other resin with a polysiloxane (refer to Japanese Unexamined Patent Publications JP-A 6-89038 (1994) and JP-A 7-114191 (1995)), and a photoreceptor intending to compatibilize the electric durability and the mechanical durability by the combination of a polyarylate resin or a polyester resin having a structure similar with the polyarylate resin, and a specific charge transportation substance (refer to Japanese Unexamined Patent Publications JP-A 10-268535 (1998), and JP-A 2001-215741).
However, it has not yet been obtained such a photoreceptor as capable of satisfying both the requirement for further higher mechanical strength in view of the digitalization and increased resolution of the electrophotographic apparatus and the requirement for the long time stabilization of electric characteristics in view of the demand for the longer life of the photoreceptor.
The charge transportation substances must satisfy the following requirements:    (1) being stable to light and heat;    (2) being stable to ozone, nitrogen oxides (NOx) and nitric acid that may be generated in corona discharging on a photoconductor;    (3) good charge transportation ability;    (4) being compatible with organic solvents and binder resins;    (5) being easy to produce and are inexpensive. Though partly satisfying some of these, however, the charge transportation substances could not satisfy all of these at high level.
Further, in a case where the charge transportation layer in which a charge transportation substance is dispersed in the binder resin forms the surface layer of the photoreceptor, particularly high charge transportation ability is required for the charge transportation substance.
An electrophotographic apparatus such as a copying machine or a laser beam printer comprises a photoreceptor, charging means such as a charging roller for charging a surface of the photoreceptor to a predetermined potential, exposure means for subjecting the charged surface of the photoreceptor to exposure to light, developing means for supplying a developer containing a toner by a magnetic brush or the like to the surface of the photoreceptor and developing means for developing electrostatic latent images formed by exposure, transfer means for transferring the toner images obtained by development onto a recording medium, fixing means for fixing the transferred toner images, and cleaning means for removing the toner remaining on the surface of the photoreceptor by a cleaning blade or the like after the transferring operation by the transferring means thereby cleaning the surface of the photoreceptor. In a case where, the photoreceptor is used being mounted on an electrophotographic apparatus, the surface layer of the photoreceptor is obliged to be partially scraped off by a contact member such as a cleaning blade or a charging roller. In a case where the scraping amount of the surface layer of the photoreceptor is large, the charge retainability of the photoreceptor lowers and images of good quality can no more be provided for a long period of time. Accordingly, for improving the durability of the electrophotographic apparatus such as the copying machine or the laser beam printer, it has been demanded a photoreceptor with high resistance having a surface layer resistant to the contact member, that is, a surface layer of high printing resistance with less amount scraped by the contact member.
In order to improve the durability of the photoreceptor by strengthening the surface layer, it may be considered to increase the content of the binder resin in the charge transportation layer as the surface layer. However, as the content of the binder resin in the charge transportation layer increases, the light responsivity lowers. In a case where the light responsivity is lowered, that is, the decay speed of the surface potential after exposure is slow, since it is used repeatedly in a state where the residual potential increases and the surface potential of the photoreceptor is not sufficiently decayed, the surface charges at the portion to be erased by the exposure are not erased sufficiently to result in troubles such as early lowering of the image quality. It is known that the light responsivity depends on the charge mobility of the charge transportation substance, and the lowering of the light responsivity is attributable to the low charge transportation ability of the charge transportation substance. That is, along with increase of the content of the binder resin, the charge transportation substance in the charge transportation layer is diluted to further lower the charge transportation ability of the charge transportation layer to lower the light responsivity. Accordingly, in order to prevent lowering of the light responsivity and ensure a sufficient light responsivity, a particularly high charge transportation ability is required for the charge transportation substance.
Further, the size has been reduced and the speed has been increased in electrophotographic apparatus, for example, in digital copying machines and printers in recent years, and improvement for the sensitivity has been required as the characteristics of the photoreceptor for coping with the increase of the speed, and high charge transportation ability has been demanded more and more as the charge transportation substance. Further, in the high speed electrophotographic process, since the time from exposure to development is short, a photoreceptor of high light responsivity is demanded. As described above, since the light responsivity depends on the charge transportation ability of the charge transportation substance, a charge transportation substance having a higher charge transportation ability is demanded also with such a view point.
As the charge transportation substance capable of satisfying such a demand, an enamine compound having a charge mobility higher than that of the charge transportation substance described above has been proposed (refer, for example, to Japanese Unexamined Patent Publications JP-A 2-51162 (1990), JP-A 6-43674 (1994) and JP-A 10-69107(1998)).
Further, a photoreceptor provided with a high charge transportation ability by the incorporation of a polysilane and improved with the chargeability and the film strength by the incorporation of an enamine compound having a specific structure has been proposed (refer to Japanese Unexamined Patent Publication JP-A 7-134430 (1995)).
On the other hand, the performance such as the durability of the function separation type photoreceptor greatly depends on the binder resin itself.
For the binder resin used for the charge transportation layer of the function separation type photoreceptor, it has been well-known that a bisphenol A polycarbonate resin using 2-bis(4-hydroxyphenol)propane (common name: bisphenol A) represented by the following structural formula (A) as a raw material provides favorable characteristics in view of the charge ability, the sensitivity, the residual potential, and the repetitive performance (refer, for example, to Japanese Unexamined Patent Publication JP-A 5-61215 (1993), page 4).

Further, it has been proposed a technique of improving the durability by incorporating a bisphenol Z polycarbonate resin using 1,1-bis(4-hydroxyphenol)cyclohexane (common name: bisphenol Z) as a raw material for the binder resin to the surface of the photosensitive layer (refer, for example, to Japanese Examined Patent Publication JP-No. 2844215).
However, the bisphenol A polycarbonate resin used for the photoreceptor described, for example, in JP-A 5-61215 involves the following drawbacks that are attributable to the structural symmetry of bisphenol A.    (1) It is poor in the solubility and shows favorable solubility only to some halogen type organic solvents such as dichloromethane or 1,2-dichloroethane. Since the halogen type organic solvents described above have low boiling point, when a photoreceptor is manufactured by using a coating solution prepared with such a solvent, since the evaporation speed of the solvent is excessively high, so that the coating film tends to be clouded due to the heat of evaporation. Further, since the halogen type organic solvent such as dichloromethane or 1,2-dichloroethane gives a significant effect such as high toxicity and destruction of ozone layers on an operator or on the global environment, administration for manufacturing steps are complicated.    (2) The resin is soluble partially to other halogen type organic solvents than those described above such as tetrahydrofurane, dioxane or cyclohexane, or mixed solvents thereof, but the coating solutions prepared with the solvents described above are poor in the aging stability such that they gel within several days after preparation. Particularly, in a case of manufacturing a photoreceptor by a manufacturing method such as dip coating, the coating solution in the coating tank gels to sometimes bring about a trouble in the production of the photoreceptor.    (3) Since the inter-molecular attraction force of the resin per se is strong, the formed coating film is poor in the adhesion and tends to suffer from crackings form the boundary with other layers. Further, since the close bondability is poor, the potential barrier layer formed near the boundary increases, so that charges generated from the charge generation substance can not be transferred smoothly as far as the surface of the photosensitive layer and, in a case where the photoreceptor is used continuously, the difference between the bright area potential as the surface potential for the exposed portion and the dark area potential as the surface potential for the not exposed area is decreased. Accordingly, fogging of formed images increases in a case of normal development, while the image density lowers in a case of reversal development, failing to form good images.    (4) Since the crystallinity of the resin per see is high, a polycarbonate resin crystallized to the surface of the film tends to precipitate to cause protrusion during formation of the coating film. Accordingly, tailing is caused in the coating film to lower the productivity. Further, the toner is deposited to the protruded portions during use of the photoreceptor, which remain without cleaning tending to cause image defects due to so-called cleaning failure.    (5) Since the resin itself lacks in the mechanical strength, the photoreceptor using the bisphenol A polycarbonate resin as the binder resin tends to surfer from injuries at the surface by being frictionally rubbed with a charge roll, a magnetic brush, or a cleaning blade and is gradually abraded.
Further, as the characteristics of the photoreceptor, it has been demanded that the light responsivity does not lower even in a case of use under a low temperature circumstance and change of characteristics is small and reliability is high also under various circumstances. However, while the photoreceptor using the bisphenol Z polycarbonate resin as the binder resin described in Japanese Patent No. 2844215 has favorable resistance to printing and wear resistance, it has low light responsivity and, particularly, the responsivity lowers when used under a low temperature circumstance to bring bout a problem that the quality of the formed images is deteriorated.
In order to suppress the lowering of the light responsivity under such a low temperature circumstance, it may be considered to use a charge transportation substance of high charge mobility as described above. However, no sufficient light responsivity can be obtained under the low temperature circumstances even using an enamine compound of high charge mobility used for photoreceptors described in JP-A 2-251162, JP-A 6-43674, or JP-A 10-69107 above. Further, while the photoreceptor described in JP-A 7-134430 is provided with a high charge transportation ability by the incorporation of polysilane, the photoreceptor using the polysilane involves a problem that it is sensible to light exposure and that various characteristics of the photoreceptor are deteriorated by exposure to light, for example, during maintenance.
In the image forming apparatus forming images by electrophotography, images are formed by way of an electrophotographic process as described below. At first, after supplying a predetermined charge potential from charging means provided to the apparatus to the surface of an electrophotographic photoreceptor (hereinafter simply referred to also as “photoreceptor”), thereby charging the surface to a predetermined potential, light is irrigated in accordance with image information by the image exposure means to subject the surface to exposure to light thereby forming an electrostatic latent image. A developer containing a toner, etc. is supplied from the developing means to the thus formed electrostatic latent images to visualize the toner images. The thus formed toner images are transferred from the surface of the photoreceptor to a recording medium such as paper by the transfer means and then they are fixed by the fixing means.
As the charging means, a charging device of corona charging system supplying a charge potential from a wire electrode to the surface of a photoreceptor by corona discharge is generally used. However, since charging is conducted in a no-contact manner in the charging device of the corona charging system, the charging efficiency to the surface of the photoreceptor is low, and a higher potential compared with the charge potential on the surface of the photoreceptor has to be applied to the wire electrode. For example, in order to charge the surface of the photoreceptor to negative (−)700 V, a voltage at about negative (−)5 kV to negative (−)6 kV has to be applied to the wire electrode. Accordingly, a large power source device is necessary, which brings about a problem of increasing the cost. Further, since a great amount of ozone is generated by corona discharge in the charging device of corona charging system, this also brings about a problem that the material constituting the photoreceptor tends to be denatured to degrade images or give undesired effects on human bodies.
In view of the above, a contact type charging device for supplying the potential directly by contacting the charging member to the surface of the photoreceptor has been developed in recent years. For example, it has been proposed a charging device using a composite material in which an electroconductive material such as electroconductive particles is dispersed in an insulative elastic material is bonded to the surface of a metal core formed in a roller shape as a charging member (for example, refer to Japanese Unexamined Patent Publications JP-A 58-49960 (1983), JP-A 63-170673 (1988), JP-A 63-149669 (1988), JP-A 64-73365 (1989), and JP-A 1-172857 (1989)). The composite material is formed such that the volumic resistance is about from 106 to 107 Ωcm and, by the application of a voltage to the metal core in a state of contacting the portion of the composite material to the surface of the photoreceptor, a potential is supplied by way of the electroconductive particles to the surface of the photoreceptor. As the insulative elastic material, a polymeric material such as silicone rubber, polyurethane rubber, ethylene-propylene-diene copolymer (simply referred as EPDM) rubber, or nitrile rubber is used. As the electroconductive particles, carbon powder, carbon fiber, metal powder, or graphite is used, for example.
Charging by the contact type charging device is conducted, specifically, by gap discharge generated in a minute gap between the charging member and the photoreceptor. The gap discharge is generated by applying a voltage at a certain value or higher between the charging member and the photoreceptor. That is, charging is started by applying a voltage above a charge threshold value voltage as a voltage for generating gap discharge between the charging member and the photoreceptor. Accordingly, when the photoreceptor is charged, a voltage at a predetermined value equal with or higher than the discharge threshold value voltage, for example, about 1 to 2 kV is applied to the charging member.
While the voltage is generally a DC voltage, in a case where only the DC voltage is applied to the charging member, it is difficult to attain a desired value of the surface potential on the photoreceptor. This is attributable to that the charging becomes not uniform due to the fluctuation of the charging voltage by the fluctuation of the resistance value of the charging member caused by the fluctuation of ambient temperature or humidity of the apparatus or change of the film thickness of the photosensitive layer caused by scraping of the photoreceptor during repetitive use. Then, in JP-A 63-149669, JP-A 64-73365, and JP-A 1-172857, a vibrating voltage formed by superposing an AC component having a peak-to-peak voltage higher by twice or more the discharge threshold value voltage to the DC component corresponding to the desired charging voltage is applied to the charging member with an aim of uniform charging. By the application of the vibrating voltage, when the surface potential on the photoreceptor rises to a value higher than the DC component of the vibrating voltage, since excess charges on the surface of the photoreceptor can be transferred backwardly from the photoreceptor to the charging member, it is possible to suppress the effect by an external factor such as the environment or film scraping of the photoreceptor and converge the surface potential of the photoreceptor to the DC component of the applied vibrating voltage.
On the other hand, as the photoreceptor, inorganic photoreceptors using inorganic photoconductive materials such as selenium, cadmium sulfide and zinc oxide have been used generally so far. Further, as the organic photoreceptor using the organic photoconductive material, those using a photoconductive polymer typically represented by poly(N-vinylcarbozole), those using an organic photoconductive material of low molecular weight such as 2,5-bis(p-diethylaminophenyl)-1,3,4-oxadiazole, as well as a combination of such an organic photoconductive material with various kinds of dyes and pigments are known.
Since the organic photoreceptor has a good film forming property for the photosensitive layer and can be produced by coating, it has an advantage that the productivity is extremely high and that it is inexpensive. Further, it also has an advantage that the light sensitive wavelength region can be controlled optionally by properly selecting the dyes, pigments, etc. to be used. Since the organic photoreceptors have many advantages as described above, they have been studied extensively. Particularly, the sensitivity and the durability which are concerned with the drawbacks on the existent organic photoreceptor have been remarkably improved recently by the development of a function separation type photoreceptor having a photosensitive layer in which a charge generation layer using an organic photoconductive dye or pigment as a charge generation substance and a charge transportation layer containing a photoconductive polymer or an organic photoconductive material of low molecular weight as a charge transportation substance are stacked, and the organic photoreceptor has become predominant in the electrophotographic photoreceptors.
However, since defects such as agglomerated portions of the charge transportation substance and the charge generation substance are tended to occur in the organic photoreceptor, when charging is conducted by using the contact type charging device described above to the organic photoreceptor, it results in the following problems. That is, in the contact type charging device, since a high electric field is applied being concentrated to the contact portion between the photosensitive layer and the charging member, charges from the charging member are concentrated to the defective portions, if any, in the photosensitive layer to charge the photosensitive layer not uniformly to cause spotwise or stripe-like image defects. Further, in a case where charges are concentrated remarkably from the charging member to the defective portions, leakage occurs to the photosensitive layer and the photosensitive layer itself suffers from dielectric breakdown and subsequent formation of normal images can no more be conducted. Further, the charging member itself undergoes damages by the leak current and it can be used no more.
As the technique for solving the problem caused by the leakage in the photosensitive layer, it has been proposed, for example, coating a coating solution divisionally for plural times upon forming the charge transportation layer by coating, thereby decreasing the overlap of the defects in the direction of the film thickness of the charge transportation layer (refer to Japanese Unexamined Patent Publication JP-A 10-10761 (1998)), and suppression of agglomeration of the charge transportation substance by decreasing the amount of the charge transportation substance to the binder resin in the photosensitive layer (refer to Japanese Unexamined Patent Publication JP-A 2001-56595).
Further, while corona discharge or gap discharge is utilized for charging the photoreceptor as described above, the organic photoreceptor involves a problem that the charge transportation substance tends to cause decomposition or degradation of the charge transportation substance by active gases such as ozone or NOx generated by the discharge, tending to degrade the surface of the photosensitive layer and electric characteristics such as the chargeability, the sensitivity and the responsivity are lowered due to the repetitive use to degrade the picture quality. In a case of using the contact type charging device as the charging device, since discharge occurs near the surface of the photoreceptor, degradation on the surface of the photoreceptor caused by discharge is more serious than in a case of using the charging device of corona discharging system. Further, in a case of applying the vibrating voltage to the charging member for uniform charging, discharge occurs also upon reversed transfer of the excess charges on the surface of the photoreceptor to the charging member as described above and the discharge occurs more frequently compared with the case of applying only the DC voltage, degradation of the surface of the photoreceptor is more conspicuous.
Further, in a case of using the contact type charging device, since the surface of the photosensitive layer is scraped by the contact with the charging member, the photosensitive layer suffers from more wearing due to repetitive use compared with the case of using the charging device of corona charging system. In a case where the amount of wear of the photosensitive layer is large, the charge retainability is lowered and images of high quality can no more be provided. Further, when the thickness of the photosensitive layer is thus decreased, dielectric breakdown of the photosensitive layer described above tends to generate further.
For suppressing the degradation and wear on the surface of the photosensitive layer, it has been proposed to use a charge transportation layer formed by polymerizing a hole transporting compound having two or more chain polymerizable functional groups in one identical molecule. According to the technique, since the portion that functions as the charge transportation substance is contained in the polymerized hole transferring compound and does not agglomerate, occurrence of defects to the photosensitive layer can be suppressed (refer to Japanese Unexamined Patent Publication JP-A 2001-166502).
In the technique described in JP-A 10-10761, since the occurrence of defects per se can not be suppressed, dielectric breakdown of the photosensitive layer can not be avoided. Further, since it is necessary to repeat the step of coating the coating solution and the step of drying the same for forming the charge transportation layer in this technique, the production efficiency is poor.
Further, in the technique described in Japanese Unexamined Patent Publication JP-A 2001-56595, the sensitivity and the responsivity of the photoreceptor are insufficient and, in a case of a high speed electrophotographic process, image defects such as background stains and lowering of the image density occur.
Further, in the technique described in Japanese Unexamined Patent Publication JP-A2001-166502, it is necessary to polymerize the hole transferring compound by radiation rays or the like in order to form the charge transportation layer of the photoreceptor and this is difficult to manufacture by the existent manufacturing apparatus.